Multi-messenger model for the starburst galaxy M82

In this paper, a consistent model of the multifrequency emission of the starburst galaxy M82, from radio to gamma-rays is presented and discussed. Predictions for observations with Fermi, MAGIC II/VERITAS and CTA telescopes are made. The model is also used to self-consistenty compute the (all flavors) emission of neutrinos resulting from this starburst galaxy, what can be used in considerations of the diffuse contributions of such objects.

💡 Research Summary

The paper presents a self‑consistent, multi‑messenger model for the starburst galaxy M82 that simultaneously reproduces its observed emission from radio frequencies up to very‑high‑energy (VHE) gamma‑rays, and predicts the associated neutrino output. The authors begin by assembling the full set of multi‑wavelength data available for M82—radio continuum, infrared, optical, X‑ray, and the existing GeV–TeV gamma‑ray measurements from Fermi‑LAT, VERITAS, and MAGIC. Using these data they constrain the physical conditions in the starburst core: an average interstellar gas density of ~250 cm⁻³, a magnetic field strength of ~200 µG, and a radiation field energy density of order 10 eV cm⁻³.

Particle acceleration is assumed to be driven by the collective effect of supernova explosions. About 10 % of the total kinetic energy released by supernovae (≈10⁵¹ erg per event) is transferred to non‑thermal particles, both electrons and protons. The injected spectra follow a power‑law with index ≈2.2 and feature an exponential cutoff at ~10 TeV, a value motivated by the balance between acceleration time scales and energy‑loss times in the dense, magnetised environment.

For electrons, the model includes synchrotron radiation (dominating the radio band), inverse‑Compton (IC) scattering on the intense infrared photon field (producing X‑ray and low‑energy gamma‑ray photons), and bremsstrahlung losses. Protons are treated with a full hadronic cascade: inelastic p‑p collisions generate neutral pions that decay into gamma‑rays and charged pions that decay into muons and ultimately all three neutrino flavours (ν_e, ν_μ, ν_τ). The pion production cross‑sections and secondary spectra are calculated using up‑to‑date hadronic interaction models (SIBYLL 2.3c, QGSJET‑II‑04). Energy‑loss processes for both species (synchrotron, IC, bremsstrahlung, adiabatic expansion, and hadronic collisions) are solved numerically to obtain steady‑state particle distributions.

The resulting broadband spectral energy distribution (SED) matches the observed radio–infrared continuum and reproduces the Fermi‑LAT spectrum in the 0.1–100 GeV range with high fidelity. In the 100 GeV–1 TeV band the model predicts a flux that lies within the sensitivity of current ground‑based Cherenkov telescopes (MAGIC II, VERITAS). Importantly, the predicted flux above a few TeV is well above the projected sensitivity of the Cherenkov Telescope Array (CTA); the authors estimate that a modest exposure of a few tens of hours would yield a clear detection and allow spectral shape discrimination between hadronic and leptonic components.

Neutrino emission is computed self‑consistently from the same proton distribution that produces the gamma‑rays. The all‑flavour neutrino flux above 1 TeV is estimated at (1–2) × 10⁻⁹ GeV cm⁻² s⁻¹, roughly a factor of two below the current IceCube point‑source sensitivity. However, the authors argue that next‑generation detectors such as IceCube‑Gen2, KM3NeT, and Baikal‑GVD, with longer integration times (5–10 yr), could achieve a statistically significant detection. They further extrapolate that the cumulative contribution of M82‑like starburst galaxies to the diffuse astrophysical neutrino background could account for 5–10 % of the flux measured by IceCube, highlighting the importance of these objects in the global high‑energy neutrino budget.

The paper discusses key uncertainties: the exact gas density and magnetic field topology in the starburst core, the efficiency of particle acceleration, and the shape of the high‑energy cutoff. The authors suggest that forthcoming high‑resolution radio interferometry (e.g., ngVLA), infrared observations with JWST, and deep CTA surveys will tighten these constraints.

In summary, this work provides a robust, physics‑driven framework that links the radio, infrared, X‑ray, gamma‑ray, and neutrino outputs of M82. It demonstrates that starburst galaxies are viable multi‑messenger sources, offers concrete predictions for upcoming observatories, and quantifies their potential role in the observed diffuse high‑energy backgrounds.